The exhaust gas from automobiles contains environmental pollutants including unburned matter such as CO (carbon monoxide) and HC (hydrocarbon), and NOx (nitrogen oxide), or the like.
In an automobile, as disclosed in Japanese Unexamined Patent Application No. Hei-7-174017, catalysts for removing unburned matter such as CO and HC, and NOx are provided in an exhaust gas conduit for preventing the unburned matter such as CO and HC, and NOx from being discharged into the atmospheric air.
However, the catalysts are degraded due to use for a long period of time, and thus, the efficiency for removing the unburned matter such as CO and HC, and NOx is lowered. Therefore, it is not possible to stably remove the unburned matter such as CO and HC, and NOx. Further, since the degraded catalysts need to be replaced periodically, maintenance cost for replacement operation is required.